I used to have a regular 28 day cycle, but since having my first child, my cycle is from 25 to 28 days. How can I tell when I am ovulating?

The only way of knowing for sure is either by using ultrasound ovarian follicle tracking-usually offered in an assissted conception or gynaecological ultrasound unit, or-more simply-by using a urine ovulation kit. You can dip your urine every day just before and after mid-cycle, and it'll show you when the lutienizing hormone (LH) surge occurs; this is abut 24 hours before ovulation. Less reliable but cheaper methods include checking your cervical mucus, and keeping a daily temperature chart.